1. Field of the Invention
The present invention relates to phosphor used in combination with light-emitting diode chips that emit blue or ultraviolet light, and more specifically β-Sialon comprising Eu2+ in a form of a solid solution applicable to white LED and various light-emitting devices, its applications, and its producing method. Hereinafter, the light-emitting diode is referred to as LED.
2. Description of the Related Art
With the increase in output of white LED, demands for heat resistance and durability of phosphor used for them are increasing. In particular, as phosphor having little brightness variation due to temperature increase and excellent durability, those using highly covalent nitride and oxynitride as base materials are attracting attentions.
Of nitride and oxynitride phosphor, divalent europium (Eu) ion doped β-Sialon which contains divalent europium (Eu) ions in a form of solid solution, in particular, is coming into practical use as a phosphor useful for white LED because it is excited by light in wide wavelength range from ultraviolet to blue light and emits green light having its peak in the wavelength range from 520 to 545 nm (Patent Literature 1).
In Eu2+ doped β-Sialon, aluminum (Al) replaces the position of silicon (Si), and oxygen (O) replaces the position of nitrogen (N), of β-type silicon nitride crystal, as solid solutions, and is given by general formula Si6-zAlzOzN8-z (z: 0 to 4.2) because there exist two formula-weight atoms in a unit cell (unit lattice). Eu ions, namely light emission center, do not replace the positions of Si and Al but are present in a form of solid solution by entering channel-shaped gaps that extend in the direction of c-axis of the β-type silicon nitride crystal (Non-patent Literature 1). Luminescence of divalent Eu ions is caused by 4f-5d transition, and emission property depends largely on the environment where Eu ions are present. Consequently, it is possible to adjust the emission property based on the z value, which is a β-Sialon composition parameter, and Eu ion concentrations. With the conventional Eu2+ doped β-Sialon, however, the following problem remains unsolved: if its composition and the amount of Eu to be added are adjusted, luminescence intensity reaches the maximum when the peak fluorescence wavelength falls within the range from 540 to 545 nm, and if the peak fluorescence wavelength falls outside this range, the luminescence intensity shows a steep decline (Non-patent Literature 2).